This invention relates to improvements in motor control circuits for use in electric power assisted steering systems, and in particular to pulse width modulation (PWM) control of multiple phase brushless motors in electric power assisted steering systems.
A motor control circuit for a typical PWM controlled multi-phase electric motor, especially a DC motor, comprises a switching circuit including a set of switches that selectively connect each phase to a voltage source and a controller that modulates the switches to apply PWM voltages to the phases of the motor. The current through the windings or phases of the motor is measured by means of separate current sensors for each of the phases, or by means of a single current sensor that is placed in the circuit so as to measure the total instantaneous current flowing between the D.C. power supply and the switching circuit and motor combination. In a single current sensor system, the multiple motor phase currents are derived by offsetting the PWM patterns of the switches which apply the required voltage to each phase, and sampling the current sensor at appropriate points.
The measured currents are typically converted into the d-q frame with reference to the rotor position and then compared with a current demand signal, also in the d-q frame, indicative of the current that is demanded from the motor, to produce an error signal. Where the controller is primarily controlling the torque output from the motor, the demand current is often generated as a function of a torque signal and the known motor properties. For example, where the motor is used in an electric power assisted steering system the torque demand signal is principally a measure of the amount of assistance torque the motor should apply to the steering to help the driver to turn the wheel.
The error signal therefore represents the difference between the current that is demanded and the actual current flowing in the motor. The error signal is fed to a current controller which produces a set of voltage demand signals representative of the voltages to be applied to each phase of the motor that will best drive the error signal towards zero, and so best ensure that the demanded current flows in the motor. The current controller typically converts the d-q error signal into a d-q voltage demand signals which are then converted further into the three phase voltage signals. These three voltage signals are then converted into respective PWM signals for each of the motor phases depending on which PWM strategy is used. The current controller therefore acts to vary the PWM phase voltages in order to try to constantly minimise the magnitude of the error signal thereby ensuring that the motor current is as close as possible to the demanded current.
In a practical system the current controller will comprise a PI or PID or other type of feedback controller.
Motor drive circuits using feedback control and PWM are well known in the art. For example, WO2006005927, discloses a typical system and the teaching of that document is incorporated herein by reference in entirety.
During steady state operation of the motor, at constant torque and speed, the voltages applied to each of the phases of a three phase motor are typically chosen so that the current in each phase varies sinusoidally over an electrical revolution of the motor rotor, the frequency of the signal therefore being dependent on the speed of rotation of the rotor of the motor, and the magnitude of which is dependent on the required torque. By arranging for each of the current waveforms to be offset from the others by 120 degrees as shown in FIG. 3 the overall current carried by the motor will be constant and so the motor torque will also be constant as the motor rotates. Applying currents in this pattern ensures that the motor rotates smoothly with no peaks in torque. This is well known in the art and the theory behind this will therefore not be described in detail here.
It is also known to provide a fault mode of operation for the motor in the event that one of the phases of the motor fails in an open circuit. When this fault occurs, current can only flow in the two remaining motor phases. If the controller continues in a normal mode of operation to try to apply the waveforms that would be used in normal operation, as shown in FIG. 4, the torque of the motor will vary as the motor rotates because the overall current will vary. In certain applications, such as electric power steering, this variation will be felt through the steering wheel as a torque ripple which may worry a driver. This is shown in Figure